This invention relates to transmission devices utilized to transmit torque between driving and driven members, and more particularly to transmission devices including a field responsive fluid for increasing the amount of torque transferred.
Field responsive fluids are capable of changing their properties when acted upon by an external field. In particular, field responsive fluids can exhibit dramatic changes in their rheological behavior in response to an external field. Typically, field responsive fluids demonstrate an increased viscosity when an external field is applied.
Field responsive fluids, as the term is utilized in this patent, include electro-rheological fluids, magneto-rheological fluids and ferro-fluids.
Ferro-fluids contain fine ferrite particles, usually of from 5 to 50 nanometers in size, suspended in a base fluid. The base fluid is usually a lubricant that can be mineral or synthetic oil, or a combination thereof including other additives. In the absence of a magnetic field, the properties of the ferro-fluid are the same as the properties of the base fluid without the ferrite particles. When a magnetic field is applied, the rheological properties of the ferro-field change dramatically, usually exhibiting an increased viscosity. Due to the small size of the ferrite particles, ferro-fluids can penetrate almost all lubricated contacts of a transmission device. Also, ferrite particles having the sizes outlined above are below the size of a single magnetic domain, such that the fluid does not retain the magnetic field and become a permanent magnet. Therefore, particles within the ferro-fluid exhibit magnetic properties only when they are in a magnetic field or they are acted upon by a magnetic field. When the field is removed, the magnetism affects are removed and the properties of the ferro-fluid return to the values characteristic of the base fluid itself. The ferrite particles do not coagulate to form agglomerates as may occur when utilizing a fluid having magnetic particles of a size larger than the size of a magnetic domain.
Traction drive continuously variable transmissions (TCVTs) are mechanisms known in the art to smoothly change the ratio of angular velocities of the engine and the wheels of a vehicle. The TCVT performs similar functions to that of a standard car (geared or stepped transmission) with the main difference being that the transmission ratio of the standard transmission is changed in steps due to the engagement of different sets of gears, while the TCVT allows changing this ratio smoothly and continuously without steps. The TCVT contains at least one pair of rolling elements that transmits the rotation from a car engine to the wheels. The rolling elements usually are installed with the ability to move either angularly or linearly or both with respect to each other. The change in the transmission ratio is achieved by moving one rolling element with respect to the other either linearly or angularly. A typical example of a TCVT design includes a cone shaped roller engaged with a toroidal shaped roller. In this arrangement, the rotational axis of the rollers are parallel and both rollers are enclosed in a jacket that is partially filled with a traction fluid. The toroidal shaped roller can slide along the axis of the cone-shaped roller resulting in a change of the contact radius of the cone-shaped roller. The transmission ratio is proportional to the ratio of the contact radii of the rollers such that a change in the contact radius results in a transmission ratio change.
Shear stress and torque is transmitted in a TCVT through a small contact patch between the rolling elements. If the deformations in contact between the two rolling bodies are neglected, the contact mathematically occurs in a single point. However, the bodies do naturally deform when in use and the theoretical contact point between them transforms into a contact patch. The dimensions and the pressure distribution within the contact patch are generally determined by Hertzian formulas, commonly known in the art [See, for example, K. L. Johnson, xe2x80x9cContact Mechanicsxe2x80x9d, Cambridge University Press, N.Y., N.Y., 1989]. The contact forces are usually large while the area of contact is usually small. The resulting pressure in the contact patch area is very high with common pressures of up to 4 gigapascals. The rolling elements, as described above, are partially submerged in a standard transmission fluid which is drawn into the contact patch. Under the very high contact pressure, the fluid is partially squeezed out between the contact surfaces leaving a thin continuous fluid film separating the two rolling bodies.
When one of the rolling bodies is a driver, driving another rotating body, as is the case of a TCVT, a shear force through the contact patch transmits the torque. The thin fluid film described above separating the contacting bodies carries this shear force. Therefore, the torque that the TCVT can transmit is determined by the properties of this fluid film and is limited by the shear strength of the fluid in the contact patch. Therefore, it is desirable to utilize a fluid that has the highest shear strength and thus provides the transmission with the ability to transmit the highest torque with other properties being equal.
It is known in the art that under high pressure such as that between the pair of rollers, a fluid undergoes a phase transformation resulting in changes of its rheological properties. In particular, the viscosity of the fluid increases such that the fluid acquires some of the properties of a solid, such as the ability to transmit shear stress. Special fluids known in the art that provide large viscosity gain due to contact pressure have been developed for application in TCVT applications. However, known fluids cannot perform well at elevated temperatures such as 150xc2x0 Centigrade which may develop in a transmission of a vehicle. The known fluids show a decrease in viscosity at elevated temperatures, resulting in a drop in the shear strength of the fluid. The resulting drop in shear strength leads to a drop in the amount of torque that may be transmitted by the transmission.
Mathematically, the ability of a fluid film to transmit shear stress is characterized by a traction coefficient, which is the ratio of the largest shear force that can be developed in the contact to the normal force in the same contact. The value of the traction coefficient is dependent upon the temperature, relative speeds of the elements, as well as other parameters. The best performing traction fluids known in the art usually exhibit a traction coefficient below 0.12 at ambient temperature with a resulting drop to below 0.06 at temperatures of 150xc2x0 centigrade. There is, therefore, a need in the art to provide a traction fluid that shows an increase in the absolute value of the traction coefficient at elevated temperatures above 150xc2x0 centigrade.
Geared transmissions may also benefit from the use of a field responsive fluid. Gears in a vehicle transmission are often subjected to large pressure acting on the teeth of the gears. Severe conditions of operation of a transmission often require high viscosity oil lubricants to prevent surface wear and fatigue. However, the presence of clutch devices within a geared transmission often limits the possible viscosity of transmission lubricants. Often, high viscosity lubricants may cause friction clutches, commonly utilized to engage various gears associated with the transmission, to slip. It is expensive and often not practical to utilize different fluids to lubricate different components of a transmission device such as clutches and gears. Such a design would require two separate fluid systems separated from each other but having redundant components.
Therefore, in an effort to improve the gear lubrication, a field responsive fluid, such as a ferro-fluid may be utilized to vary the rheological properties of the fluid such that the fluid may be utilized as a lubricant by a number of different components that require different rheological characteristics.
A transmission device including a driving member, and a driven member that is in frictional engagement with the driving member. A traction fluid is present at a contact between the driving and driven members. The traction fluid is a field responsive fluid that is capable of changing its rheological properties in response to an external field.
In one embodiment, the transmission device includes a driving and driven member in rotational frictional engagement, as commonly found in a continuously variable transmission. The transmission device of the first embodiment has the advantage of providing a transmission that is capable of transferring a higher torque than those utilizing conventional traction fluids.
The transmission device of the first embodiment has the further advantage of including a traction fluid that has a traction coefficient that is higher than that of conventional traction fluids at elevated temperatures.
In another embodiment the transmission device includes intermeshed gears comprising the driving and driven members. The transmission device of second embodiment has the further advantage of providing a device that provides increased lubrication of gears by locally increasing the viscosity of a transmission fluid. Also, the transmission device of the second embodiment has the advantage of utilizing a traction fluid that can vary its rheological characteristics to lubricate various components of the transmission device having different optimal rheological characteristics.